The present invention relates generally to the field of robotics and specifically to a safety interlock provided on the tool side of a robotic tool changer.
Industrial robots have become an indispensable part of modem manufacturing. Whether transferring semiconductor wafers from one process chamber to another in a cleanroom or cutting and welding steel on the floor of an automobile manufacturing plant, robots perform many manufacturing tasks tirelessly, in hostile environments, and with high precision and repeatability.
In many robotic manufacturing applications, it is cost-effective to utilize a relatively generic robot to accomplish a variety of tasks. For example, in an automotive manufacturing application, a robot may be utilized to cut, grind, or otherwise shape metal parts during one production run, and perform a variety of spot welding tasks in another. Different welding tool geometries may be advantageously mated to a particular robot to perform welding tasks at different locations or in different orientations. In these applications, a tool changer is used to mate different tools to the robot. One half of the tool changer, called the master unit, is permanently affixed to a robot arm. The other half, called the tool unit, is affixed to each tool that the robot may utilize. When the robot arm positions the master unit adjacent the tool unit connected to a desired tool, a coupler is actuated that mechanically locks the master and tool units together, thus affixing the tool to the end of the robot arm. Utilities such as electrical current, air pressure, hydraulic fluid, cooling water, electronic or optical data signals, and the like, may be transferred through the robot changer from the master unit to the tool unit via mating terminals, valve connections, electrical connectors, and the like, making the utilities available to the selected tool.
Safety is of paramount concern in any industrial robotic application. To prevent possible injury or damage to the tool, it is imperative that a tool not come dislodged from a robot arm to which it is coupled until the robot arm has positioned the tool in a tool stand or similar receptacle designed to safely support and store the tool. Since the only part of the robot arm and tool assembly not typically bolted together is the coupler of the tool changer, design and control of the coupler are key concerns. The coupler of a tool changer, i.e., the mechanism that selectively couples and decouples master unit and tool units, may comprise an electromechanical, hydrologic, pneumatic, or similar construction. Tool changers and their constituent couplers are well known in the robotics arts, and are commercially available, such as from the assignee, ATI Industrial Automation of Apex, North Carolina.
Although it may assume a wide variety of shapes, sizes, and modes of operation, a coupler is typically designed in a fail-safe manner, with its default state being coupled. That is, if power or pneumatic pressure is interrupted or a command interface is terminated, the master and tool units remain coupled together. This may be accomplished, for example, by spring-biasing the coupler to the coupled position, and requiring the positive application of electrical power, pneumatic pressure, or the like, to move it to the decoupled position. Also, control of the coupler during operation is carefully controlled, with robot control software typically performing myriad checks such as consulting sensors, shutting down utilities, removing applied power from the tool, and the like, prior to issuing a decouple command to the tool changer.
Typically, commands to actuate the coupler to couple or decouple the tool changer units are generated by a controller, which is typically located in the master unit. In modern robotic tool changers, this controller may conform to the DeviceNet specification promulgated by the Open DeviceNet Vendor Association (ODVA), information on which is available from odva.org. Alternatively, the controller may comply with other bus system specifications, or may be a custom-designed unit. Regardless of the specific controller, the generation and transmission of decouple commands is typically carefully controlled so as not to be inadvertently generated, causing untimely decoupling of the tool from the robot arm. Nevertheless, due to fear of software glitches, human error, and the like, it is desirable to interpose a hardware safety interlock into the decoupling circuit.
One example of such a safety interlock known in the art comprises physically breaking the connection that energizes the decoupling circuit upon command by the controller, and bringing the open circuit to external contacts on the tool changer. These contacts may then be connected to a switch located on the exterior of the tool unit or the tool itself, in such a position and manner that the switch contacts are closed by the tool stand when the tool is placed in the tool stand and securely supported. This closes the circuit, allowing the decouple signal generated by the controller to pass through the closed switch and reach the coupler, decoupling the master and tool units and removing the tool from the robot arm. When the tool is in any position other than safely stowed in its tool stand, the switch contacts remain open, and any decouple signal generated by the controller cannot reach the coupler to effect the decouple operation.
Prior art implementations located these switch contacts on the exterior of the master unit (which typically houses both the coupler and the controller), with a switch attached that extended to the vicinity of the tool, to be closed by the tool stand. In practice, this has been found to be a deficient solution. For example, it has been proven difficult to design and implement a switch on the master unit that is operative with a variety of tools, due to the different geometries that each tool presents. It has been discovered that in many applications, personnel simply connect a short-circuit connector to the contacts, thus thwarting the safety benefit of the interlock.
Moving the interlock contacts to the tool unit (by, for example, passing the unlock signal path between the master and tool units via inter-tool-changer utility connections) alleviates the necessity of a universal tool interlock switch design. Since each tool unit is typically permanently attached to a particular tool, a switch that fits the geometry of the tool may easily be designed and attached to the tool unit or to the tool itself. However, such a relocation introduces a problem: when the master and tool units are decoupled, as described above, the default position of the coupler is the coupled position. The coupler must move to the decoupled position to be able to mate the master unit with a tool unit. Yet the circuit to actuate the coupler is broken, and, regardless of the position of the tool-mounted switch, the circuit cannot be closed until the master and tool units are mated together and the utility contacts complete the circuit from the controller in the master unit, through the switch mounted on the tool, back to the coupler in the master unit.